Modbus Rtu Message Framing Structure - Danfoss VLT HVAC Basic Drive FC 100 Series Design Manual

RS-485 Installation and Set...
requests from another device, and how errors are detected
and reported. It also establishes a common format for the
layout and contents of message fields.
During communications over a Modbus RTU network, the
protocol determines:
How each controller learns its device address
Recognizes a message addressed to it
Determines which actions to take
Extracts any data or other information contained
in the message
If a reply is required, the controller constructs the reply
message and sends it.
Controllers communicate using a master-slave technique in
which only one device (the master) can initiate
transactions (called queries). The other devices (slaves)
7 7
respond by supplying the requested data to the master, or
by taking the action requested in the query.
The master can address individual slaves, or can initiate a
broadcast message to all slaves. Slaves return a message
(called a response) to queries that are addressed to them
individually. No responses are returned to broadcast
queries from the master. The Modbus RTU protocol
establishes the format for the master's query by placing
into it the device (or broadcast) address, a function code
defining the requested action, any data to be sent, and an
error-checking field. The slave's response message is also
constructed using Modbus protocol. It contains fields
confirming the action taken, any data to be returned, and
an error-checking field. If an error occurs in receipt of the
message, or if the slave is unable to perform the requested
action, the slave will construct an error message, and send
it in response, or a time-out occurs.
7.6.4 Frequency Converter with Modbus
RTU
The frequency converter communicates in Modbus RTU
format over the built-in RS-485 interface. Modbus RTU
provides access to the Control Word and Bus Reference of
the frequency converter.
The Control Word allows the Modbus master to control
several important functions of the frequency converter:
•
Start
•
Stop of the frequency converter in various ways:
Coast stop
Quick stop
DC Brake stop
Normal (ramp) stop
•
Reset after a fault trip
•
Run at a variety of preset speeds
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VLT HVAC Basic Drive Design Guide
•
•
•
The Bus Reference is commonly used for speed control. It
is also possible to access the parameters, read their values,
and where possible, write values to them. This permits a
range of control options, including controlling the setpoint
of the frequency converter when its internal PI controller is
used.
7.7 Network Configuration
To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter
8-30 Protocol
8-31 Address
8-32 Baud Rate
8-33 Parity / Stop
Bits
7.8 Modbus RTU Message Framing
7.8.1 Frequency Converter with Modbus
The controllers are set up to communicate on the Modbus
network using RTU (Remote Terminal Unit) mode, with
each byte in a message containing 2 4-bit hexadecimal
characters. The format for each byte is shown below.
Start
bit
Coding System
Bits Per Byte
Error Check Field
7.8.2 Modbus RTU Message Structure
The transmitting device places a Modbus RTU message
into a frame with a known beginning and ending point.
This allows receiving devices to begin at the start of the
message, read the address portion, determine which
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Run in reverse
Change the active set-up
Control the frequency converter's built-in relay
Setting
Modbus RTU
1 - 247
2400 - 115200
Even parity, 1 stop bit (default)
Structure
RTU
Data byte
8-bit binary, hexadecimal 0-9, A-F. 2
hexadecimal characters contained in each 8-
bit field of the message
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no
parity
1 stop bit if parity is used; 2 bits if no parity
Cyclical Redundancy Check (CRC)
Stop/ Stop
parity